Semiconductor chips are typically fabricated from single crystal silicon ingots. The single crystal silicon ingots are usually formed by the Czochralski (CZ) method. In the production of single silicon crystals grown by the CZ method, polycrystalline silicon in the form of granular polysilicon, chunk polysilicon or mixtures thereof is first melted within a crucible, such as a quartz crucible, of a crystal pulling device (also herein referred to as a “puller”) to form a molten silicon melt. During the CZ process, a single crystal silicon ingot is produced by melting an initial charge of a polysilicon source material within a quartz crucible to form a molten silicon melt. The melt and the crucible are heated until the temperature has stabilized at an equilibrium temperature. A seed crystal is dipped into the melt and withdrawn as the source melt crystallizes on the seed to form the single crystal ingot. The ingot is pulled out of the melt as it grows.
The size of the single crystal ingot is limited by the amount of the melt within the crucible. Thus, for single crystal silicon ingots grown by the CZ method, it may be desirable to begin the process with a large initial polysilicon charge in the crucible, in order to maximize the theoretical process yield. The maximum charge size that can be loaded in the crucible before initiating the process is limited by geometrical constraints such as the crucible size and the hot zone design, particularly the shape and position of thermal shields that surround the crucible. One known method to address the geometrical limitations involves adding more polysilicon to the melt after having melted the initial charge of polysilicon material. However, this method requires fitting the puller with a feeding tool able to add polysilicon to the crucible during the process, specifically after having melted at least part of the initial charge.
However, adding polysilicon to the melt has disadvantages, such as the cost of fitting the puller with a feeding tool and the need to prepare small size chips or granular polysilicon from larger polysilicon chunks. The granular polysilicon typically also contains a high concentration of hydrogen, which may expand suddenly when the granules come in contact with the molten silicon melt. Adding polysilicon to the melt may also undesirably produce splashing or splattering of molten silicon. Further, gasses trapped within the added polysilicon may create gas bubbles within the melt that become entrained within the single crystal ingot. This may lead to the formation of dislocations, voids or other defects within the single crystal ingot.
In order to address the issue of hydrogen within the polysilicon granules, known methods either add a dehydrogenation step during the production of granular polysilicon, which further increases its cost, or retrofit the puller with a heater at the bottom of the crucible in order to melt the silicon charge from the bottom rather than from the side to keep a solid surface on which the granular polysilicon can fall without touching the melt. Thus, known methods of adding polysilicon to the melt add cost, complexity and potential manufacturing defects to the CZ process.
This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.